This invention relates to the use of ultrasonic wave energy for the manipulation of particulate material in a liquid medium.
It is known that the acoustic forces from an ultrasonic standing wave acting on particles suspended in a liquid can attract individual particles to the nodes or the antinodes of a standing wave. For the purposes of the present invention it is immaterial whether the attraction is to the nodes or to the antinodes and to simplify the following description these zones of attraction will be referred to only as the nodes or the nodal fronts. Since a similar mechanism is operative for such particles as may be attracted to the antinodes, this alternative is accordingly included within the scope of the invention.
It has been proposed that these mechanisms in an ultrasonic standing wave can be employed to manipulate particulate material. For example in our European patent application No. 147032 (84.307496.4) there is described how two axially opposed ultrasonic transducers can establish a standing wave to control the movement of particles in a coaxial column of liquid interposed between the transducers, and how, by displacing the standing wave along its axis, it is possible to move the particles along the column under the influence of the standing wave.
Such a system has a number of limitations, not least being those arising from the effects of attenuation on the ultrasonic energy in its passage through the medium. This limits the effective length of the standing wave in the column before acoustic streaming weakens unduly the attachment of particles to the nodes. An alternative proposal, in our PCT application WO85/01892(PCT/GB84/00368) can avoid that problem but relies on establishing an appropriate degree of non-uniformity of the acoustic energy density in the nodal fronts of a standing wave, which can lead to complexity in the design and operation of such a system.
The present invention seeks to provide an apparatus and method that permits particles in a liquid medium to be controlled in a relatively simple manner and without being unduly limited by acoustic streaming effects.